Mask blanks

ABSTRACT

The present invention provides a mask blank which comprises a substrate made of a synthetic quartz glass and a light-shielding film laminated on a surface of the substrate and is for use in a semiconductor device production technique employing an exposure light wavelength of 200 nm or shorter, wherein the mask blank has a birefringence, as measured at a wavelength of 193 nm, of 1 nm or less per substrate thickness. According to the present invention, mask blanks suitable for use in the immersion exposure technique and the polarized illumination technique are provided.

TECHNICAL FIELD

The present invention relates to mask blanks for use in a semiconductordevice production technique employing an exposure light wavelength of200 nm or shorter. More particularly, the invention relates to maskblanks for use with a lithographic exposure tool employing an ArFexcimer laser (wavelength, 193 nm), F₂ laser (wavelength, 157 nm), orthe like as a light source.

BACKGROUND ART

In the production of semiconductor integrated circuits, lithographicexposure tools for reductively projecting and transferring a finecircuit pattern drawn in a photomask onto a wafer are extensively used.With the trend toward higher degrees of integration and higher functionsin circuits, the circuits are becoming finer and the lithographicexposure tools have come to be required to form a high-resolutioncircuit pattern image on a wafer surface while attaining a large focaldepth. The wavelengths of exposure light sources are becoming shorter.KrF excimer lasers (wavelength, 248 nm) and ArF excimer lasers(wavelength, 193 nm) are being used as exposure light sources in placeof the g-line (wavelength, 436 nm) and i-line (wavelength, 365 nm)heretofore in use.

Photomask substrates mainly used for lithographic exposure toolsemploying such exposure light sources are ones made of a syntheticquartz glass, because synthetic quartz glasses have advantages, forexample, that they have excellent transparency to light in a wide rangeof from the near infrared region to the ultraviolet region, have anextremely low coefficient of thermal expansion, and can be processedrelatively easily. Photomask substrates for, e.g., ArF excimer lasersare required to have a surface flatness of about 0.5 μm, a parallelismof about 5 μm, and a birefringence of about 4 to 10 nm/cm besidesresistance to ArF excimer laser light.

Recently, the technique of immersion exposure is known in which exposurewith a lithographic exposure tool is conducted while filling the spacebetween the projection lens of the lithographic exposure tool and thewafer with a liquid in order to attain a higher resolution with an ArFexcimer laser. The shorter the exposure light wavelength and the largerthe NA (numerical aperture) of the projection lens, the higher theresolution for the lithographic exposure tool becomes. The resolutioncan be represented by the following expressions.Resolution=[k(process coefficient)×λ(exposure light wavelength)]/NANA=n×sin θIn the expressions, n indicates the refractive index of the mediumthrough which the exposure light passes. In exposure techniquesheretofore in use, n is 1.0 because the medium is the air. In thisimmersion exposure, however, pure water, which has an n of 1.44, is usedas the medium and the lithographic exposure tool can hence attain aneven higher resolution.

Furthermore, the polarized illumination technique is known in whichpolarized lights which exert an adverse influence on resolution arediminished to thereby heighten image-forming contrast and improveresolution, in contrast to the exposure techniques heretofore in usewhich employ an exposure light composed of random polarized lightshaving various polarization directions.

The photomasks for use in such immersion exposure technique and/orpolarized illumination technique are required to have low birefringenceso as not to disorder the polarization of the exposure light whichpasses therethrough. A photomask substrate having a birefringencereduced to 2 nm/cm or less has hence been proposed (see, for example,patent document 1).

Patent Document 1: JP-T-2003-515192

In patent document 1, the birefringence of the photomask substrate isspecified. This birefringence of the photomask substrate is mainlyattributable to a residual strain in the synthetic quartz glass used asthe photomask substrate. However, in the case of a mask blank comprisinga photomask substrate and a light-shielding film laminated thereon, thebirefringence thereof is attributable also to the stress imposed by thelight-shielding film laminated on a surface of the photomask substrate.It is therefor necessary that this film stress should be taken intoaccount in regulating the birefringence of a mask blank comprising aphotomask substrate and a light-shielding film laminated on a surface ofthe substrate.

DISCLOSURE OF THE INVENTION

The invention has been achieved in view of the problems described above.

An object of the invention is to provide mask blanks suitable for use inthe immersion exposure technique and/or the polarized illuminationtechnique.

The invention provides a mask blank which comprises a substrate made ofa synthetic quartz glass and a light-shielding film laminated on asurface of the substrate and is for use in a semiconductor deviceproduction technique employing an exposure light wavelength of 200 nm orshorter, wherein the mask blank has a birefringence, as measured at awavelength of 193 nm, of 1 nm or less per substrate thickness.

The invention further provides a mask blank which comprises a substratemade of a synthetic quartz glass and a light-shielding film laminated ona surface of the substrate and is for use in a semiconductor deviceproduction technique employing an exposure light wavelength of 200 nm orshorter, wherein when a light-transmitting area of 260 nm×1,040 nm isformed in the light-shielding film, then the birefringence at thislight-transmitting area, as measured at a wavelength of 193 nm, is 1 nmor less per substrate thickness.

The invention still further provides a mask blank which comprises asubstrate made of a synthetic quartz glass and a light-shielding filmlaminated on a surface of the substrate and is for use in asemiconductor device production technique employing an exposure lightwavelength of 200 nm or shorter, wherein the substrate has abirefringence, as measured at a wavelength of 193 nm, of 0.5 nm or lessper substrate thickness, and wherein the light-shielding film has a filmstress of 800 MPa or lower.

The invention furthermore provides a mask blank which comprises asubstrate made of a synthetic quartz glass and a light-shielding filmlaminated on a surface of the substrate and is for use in asemiconductor device production technique employing an exposure lightwavelength of 200 nm or shorter, wherein the substrate has abirefringence, as measured at a wavelength of 193 nm, of 0.5 nm or lessper substrate thickness, and wherein the mask blank has a warpage amountof 2 μm or smaller.

The mask blanks of the invention have a low birefringence and aresuitable for use in the immersion exposure technique and/or thepolarized illumination technique.

BEST MODE FOR CARRYING OUT THE INVENTION

The mask blanks of the invention are constituted of a photomasksubstrate made of a synthetic quartz glass and a light-shielding filmlaminated on a surface thereof. The synthetic quartz glass constitutingthe photomask substrate can be produced, for example, by the followingmethods.

First, a silicon-containing compound as a raw material and oxygen gas,hydrogen gas, nitrogen gas, etc. are supplied to a burner made of quartzglass. The raw material is caused to undergo a hydrolysis reaction oroxidation reaction in an oxyhydrogen flame to thereby synthesize aquartz glass. Examples of this synthesis method include the directprocess and the soot process (e.g., the VAD process, OVD process, orindirect process).

The direct process is a process in which a silicon-containing compoundis subjected to flame hydrolysis at a temperature of 1,500 to 2,000° C.to synthesize SiO₂ particles and the particles are deposited on andfused to a target to thereby directly synthesize a transparent syntheticquartz glass body.

On the other hand, the soot process is a process which comprisessubjecting a silicon-containing compound to flame hydrolysis at atemperature of 1,000 to 1,500° C. to synthesize SiO₂ particles,depositing the particles on a target to thereby first obtain a poroussynthetic quartz glass body, and then heating this porous syntheticquartz glass body to a temperature of 1,400 to 1,500° C. to therebydensify it and obtain a transparent synthetic quartz glass body.

The VAD process is preferred because the reaction temperature duringsynthesis is relatively low and it is possible to relatively freelyregulate the composition and the concentration of defects. Inparticular, the low reaction temperature during synthesis has anadvantage that the synthetic quartz glass synthesized from achlorine-containing raw material such as SiCl₄ by the VAD process has alower chlorine concentration than that by the direct process. In thispoint also, the VAD process is preferred.

The raw material for the synthetic quartz glass is not particularlylimited as long as it can be gasified. Examples thereof include siliconhalide compounds such as chlorides, e.g., SiCl₄, SiHCl₃, SiH₂Cl₂, andSiCH₃Cl₃, and fluorides, e.g., SiF₄, SiHF₃, and SiH₂F₂, and halogen-freesilicon compounds such as alkoxysilanes represented by R_(n)Si(OR)_(4-n) (wherein R is an alkyl group having 1 to 4 carbon atoms and nis an integer of 0 to 3) and (CH₃)₃Si—O—Si (CH₃)₃.

Use of a chloride as a raw material gives a synthetic quartz glass whichcontains residual chlorine derived from the raw material. It istherefore preferred to use as a raw material an organosilicon compoundor fluoride which contains no chlorine. It should, however, be notedthat use of a fluoride as a raw material frequently poses a problemconcerning safety and handling because hydrofluoric acid (HF) isgenerated as a by-product during the synthesis. Consequently, the rawmaterial preferably is an organosilicon compound containing no halogen.

In the case where the VAD process is used for synthesizing a syntheticquartz glass, the concentrations of oxygen-excess defects, dissolvedoxygen molecules, and oxygen-deficient defects in the synthetic quartzglass can be regulated by several methods. Examples of the methodsinclude (1) a method in which the proportions of oxygen gas and hydrogengas in the feed gases are regulated, (2) a method in which a poroussynthetic quartz glass body is treated with a reducing substance such asa compound containing fluorine or chlorine, and (3) a method in whichthe conditions under which a porous synthetic quartz glass body isdensified to obtain a transparent synthetic quartz glass body areregulated.

Method (1) is a technique in which the proportion of hydrogen gas tooxygen gas in the feed gases is regulated to a value higher than 2 whichis the stoichiometric ratio, i.e., set at a value in the range of 2.0 to2.5, to synthesize a porous synthetic quartz glass body.

Method (2) is a technique in which a porous synthetic quartz glass bodyis heat-treated at a temperature of from room temperature to 1,200° C.in an atmosphere containing a reducing substance such as afluorine-containing compound, hydrogen gas, or CO gas. Examples of thefluorine-containing compound include CF₄, SiF₄, and SF₆. In the casewhere a compound containing fluorine or chlorine or CO gas is used, itis desired to use a mixed gas prepared by diluting such gas with aninert gas (e.g., nitrogen, helium, or argon) to a concentration in therange of 0.01 to 10 vol %, preferably 0.05 to 5 vol %, because thosegases have extremely high reducing properties. The treatment in thiscase is preferably conducted at a temperature of from room temperatureto about 1,000° C. at a pressure of the atmosphere of 1 to 101 kPa. Inthe case where hydrogen gas is used, it is preferred to conduct the heattreatment in an inert gas containing 50 to 100 vol % hydrogen gas underthe conditions of 1 to 10 atm and 800 to 1,200° C. In the treatment withany of those reducing gases, the porous synthetic quartz glass body isfirst held in a reduced-pressure atmosphere and the gas is introduceduntil the reduced pressure is elevated to a given pressure, whereby theporous quartz glass body can be evenly treated efficiently.

Method (3) is a technique in which the porous synthetic quartz glassbody synthesized is held at a temperature of 1,100 to 1,300° C.,preferably 1,200 to 1,300° C., for 20 to 200 hours in a reduced-pressureatmosphere consisting of a 100 vol % inert gas, such as helium ornitrogen, and having a pressure of from 10 Pa to 10 kPa using a graphitefurnace employing high-purity carbon in the heater and as the heatinsulator and being capable of atmosphere control or using a metalfurnace employing tungsten or molybdenum as a reflector and in theheater, and then densified by heating to 1,400 to 1,500° C. in thatatmosphere to thereby obtain a transparent synthetic quartz glass body.

The concentrations of oxygen-excess defects, dissolved oxygen molecules,and oxygen-deficient defects in the synthetic quartz glass can beregulated by conducting any one of methods (1) to (3) described above orby conducting two or more thereof in combination.

Distorted bonding structures in the synthetic quartz glass areprecursors for defects such as the E′ centers and NBOHC which aregenerated upon ultraviolet irradiation. The concentration thereofpreferably is lower. Specifically, in a laser Raman spectrum, the ratiosof the intensity of the scattering peak at 495 cm⁻¹ (I₄₉₅) and that ofthe scattering peak at 606 cm⁻¹ (I₆₀₆) to that of the scattering peak at440 cm⁻¹ (I₄₄₀), i.e., I₄₉₅/I₄₄₀ and I₆₀₆/I₄₄₀, are preferably 0.585 orlower and 0.136 or lower, respectively.

It is also preferred to reduce the sodium concentration in the syntheticquartz glass. To reduce the sodium concentration in the synthetic quartzglass to 5 ppb or lower is effective. The sodium concentration isespecially preferably 3 ppb or lower. To regulate the sodiumconcentration so that the difference between the maximum and minimumvalues thereof is 3 ppb or smaller is effective in reducing fluctuationsin birefringence in the range of exposure light wavelengths. The term“maximum value” or “minimum value” of sodium concentration herein meansthe largest value or smallest value among the found sodium concentrationvalues determined for respective points.

By regulating the synthetic quartz glass so as to have a chlorineconcentration of 10 ppm or lower, preferably to contain substantially nochlorine, the change in refractive index and decrease in transmittancewhich occur upon ultraviolet irradiation can be reduced to asufficiently low level. The chlorine concentration in the syntheticquartz glass can be determined by fluorescent X-ray spectroscopy. Thedetection limit for this analysis is 10 ppm. When the chlorineconcentration in the synthetic quartz glass exceeds the upper limit inthat range, there is a possibility that a larger decrease intransmittance and a larger change in refractive index might be caused byultraviolet irradiation.

Furthermore, by regulating the synthetic quartz glass so as to have anOH group concentration of 100 ppm or lower, preferably 50 ppm or lower,the change in refractive index and decrease in transmittance which occurupon ultraviolet irradiation can be reduced to a sufficiently low level.The OH group concentration can be determined with an infraredspectrophotometer by the method according to a document (Cer. Bull.,55(5), 524 (1976)). The detection limit for this analysis is 1 ppm. Whenthe OH group concentration in the synthetic quartz glass exceeds theupper limit in that range, there is a possibility that a larger decreasein transmittance and a larger change in refractive index might be causedby ultraviolet irradiation.

Metal impurities present in the synthetic quartz glass, such as alkalimetals (e.g., Na, K, and Li), alkaline earth metals (e.g., Mg and Ca),and transition metals (e.g., Fe, Ni, Cr, Cu, Mo, W, Al, Ti, and Ce), notonly reduce light transmittance in the range of from the ultravioletregion to the vacuum ultraviolet region, but also are causative of lightresistance deterioration. Because of this, the content of such metalimpurities preferably is as low as possible. Specifically, the totalcontent of metal impurities is preferably 100 ppb or lower, especiallypreferably 50 ppb or lower.

Hydrogen molecules can be incorporated in the synthetic quartz glass inan amount in the range of from 5×10¹⁵ to 1×10¹⁹ molecules per cm³.Hydrogen molecules in the synthetic quartz glass serve to repairparamagnetic defects such as the E′ centers and nonbridging oxygenradicals which are generated upon ultraviolet irradiation and therebyhave the effect of inhibiting the transmittance from decreasing uponultraviolet irradiation.

Fluorine can be incorporated in the synthetic quartz glass in an amountof 100 to 10,000 ppm. Fluorine is effective in diminishing unstablestructures in the synthetic quartz glass and improving ultravioletresistance. However, when the fluorine content in the glass is lowerthan 100 ppm, there are cases where unstable structures in the syntheticquartz glass are not diminishing to a sufficient level. When the glasscontains fluorine in an amount exceeding 10,000 ppm, there is apossibility that reduced defects might be generated, resulting inreduced ultraviolet resistance.

In order that the synthetic quartz glass having the compositiondescribed above might have a reduced birefringence so as to be used asan optical member, it is preferred to suitably conduct heat treatmentsfor imparting optical properties required of the optical member, such ashomogenization, molding, and annealing (hereinafter referred to asoptical heat treatments). Such optical heat treatments are conductedafter a dense and transparent synthetic quartz glass has been obtained.

Among those optical heat treatments, annealing is closely related to thebirefringence of the synthetic quartz glass to be obtained. In order toimpart a low birefringence to the synthetic quartz glass, the glass isheld at a temperature of 1,250° C. or higher for 5 hours or longer andthen gradually cooled to 1,050° C. at a cooling rate of preferably 5°C./hr or lower, more preferably 3° C./hr or lower. Although thisannealing may be conducted in air, it is effective to conduct thetreatment under vacuum. The degree of vacuum is preferably 10 Pa or alower pressure, more preferably 1 Pa or a lower pressure. That is, theannealing treatment is preferably carried out in the atmosphere having apressure of 10 Pa or lower, more preferably 1 Pa or lower.

A photomask substrate is produced from the synthetic quartz glass thusobtained. The photomask substrate preferably has such durability thatwhen it is irradiated using an Xe excimer lamp at an irradiance of 13.2mW/cm² for 20 minutes, then the decrease in light transmittance asmeasured at a wavelength of 217 nm, in terms of the difference betweenthe light transmittance before the irradiation and that after theirradiation, is 1.0% at the most.

The photomask substrate having such high durability is preferablyobtained in the following manner. Namely, as long as the syntheticquartz glass contains substantially no oxygen-excess defects andsubstantially no dissolved oxygen molecules, the decrease intransmittance and change in refractive index which occur uponultraviolet irradiation can be sufficiently reduced. That the syntheticquartz glass contains substantially no oxygen-excess defects andsubstantially no dissolved oxygen molecules means that theconcentrations as determined respectively by the detection methodsdescribed below are not higher than the detection limits.

The concentration of dissolved oxygen molecules can be determined byRaman spectrometry according to a literature (L. Skuja et al., J. Appl.Phys., Vol. 83, No. 11, pp. 6106-6110 (1998)). The detection limit forthis method is 1×10¹⁷ molecules per cm³. The concentration ofoxygen-excess defects can be evaluated based on the increase in OH groupconcentration through a heat treatment at 700 to 1,000° C. in anatmosphere comprising hydrogen gas. For example, a test piece of thesynthetic quartz glass having dimensions of 10×10×100 mm is heat-treatedat 800° C. for 100 hours in a 1-atm atmosphere consisting of 100%hydrogen gas and the increase in OH group concentration through thisheat treatment is determined with an infrared spectrophotometeraccording to the method described in a document (Cer. Bull., 55(5), 524(1976)). The detection limit for this method is 1×10¹⁶ molecules percm³.

As long as the synthetic quartz glass contains substantially no reduceddefects, the decrease in transmittance and change in refractive indexwhich occur upon ultraviolet irradiation can be reduced to asufficiently low level. That the synthetic quartz glass containssubstantially no reduced defects means that no peak attributable to SiHis observed at around 2,250 cm⁻¹ in Raman spectrometry.

With respect to the concentration of oxygen-deficient defects in thesynthetic quartz glass, the concentration thereof is reduced to 5×10¹⁴defects per cm³ or lower. Thus, the decrease in transmittance whichoccurs upon ultraviolet irradiation can be sufficiently inhibited.

The concentration of oxygen-deficient defects in the synthetic quartzglass can be determined from the intensity of the blue fluorescencewhich is emitted by ultraviolet irradiation and has a peak at around thewavelength range of 280 to 300 nm. Namely, a fiber lightguide typespectrophotometer equipped with a multi-channel photodiode (e.g.,spectrophotometer MCPD 2000, manufactured by Otsuka Electronics Co.,Ltd.) or the like is used to measure the intensity of the scatteredlight derived from ArF excimer laser light and the intensity of the bluefluorescence peak centering at around a wavelength of 280 to 300 nm.When the proportion of the intensity of the blue fluorescence peak tothe intensity of the scattered light having a wavelength of 193 nm is5×10⁻³ or lower, the concentration of oxygen-deficient defects in thesynthetic quartz glass can be judged to be within the range shown above.When that intensity ratio exceeds 5×10⁻³, this means that theconcentration of oxygen-deficient defects in the synthetic quartz glassexceeds 5×10¹⁴ defects per cm³ and there is hence a possibility that adecrease in transmittance might occur upon ultraviolet irradiation.

The relationship between that intensity ratio and the concentration ofoxygen-deficient defects was determined from the absorption bandattributable to oxygen-deficient defects and centering at 163 nm.Namely, the concentration of oxygen-deficient defects was determinedfrom the intensity of the absorption at a wavelength of 163 nm, and asynthetic-quartz-glass sample of which the concentration of theoxygen-deficient defects had been known was examined for the intensityof blue fluorescence. Thus, the relationship between the intensity ratioI of the blue fluorescence to the scattered light having a wavelength of193 nm and the concentration of oxygen-deficient defects C_(ODC)(defects/cm³) was obtained, which is represented by the followingequation.C_(ODC)=1.16×10¹⁷ ×I

That surface of the photomask substrate on which a light-shielding filmis to be laminated (hereinafter, the surface is often referred to as“pattern formation side”) preferably has a flatness of 0.25 μm or asmaller value, and the opposite side thereof preferably has a flatnessof 1 μm or a smaller value. In addition, the parallelism of the twosides preferably is 5 μm or a smaller value. The photomask substratesatisfying these requirements enables sufficient exposure precision tobe secured even when polarized illumination is used or immersionexposure is conducted.

The photomask substrate having such properties can be produced, forexample, by the following method.

A synthetic-quartz-glass plate having outside dimensions larger by atleast 10 mm than the dimensions of the photomask substrate is firstpolished and then cut into the given dimensions to thereby produce thephotomask substrate. Alternatively, a synthetic-quartz-glass platehaving a dummy processing part attached to the periphery thereof ispolished to thereby produce the photomask substrate.

Namely, a synthetic-quartz-glass plate larger by at least 10 mm than theoutside dimensions of the photomask substrate to be used is polished andfinished so as to have a given thickness, and a peripheral part is thencut off. Thus, a photomask substrate having satisfactory thicknessfluctuations is obtained. In the other method, a dummy processing parthaving the same thickness as the photomask substrate to be produced isdisposed in place of the cutting allowance on the periphery of asynthetic-quartz-glass plate, and this glass plate is set on a carrierlike the photomask substrate, whereby sagging at the periphery isdiminished.

The dummy processing part preferably has a width of 10 mm or larger. Thedummy processing part preferably is one made of a synthetic quartz glassbecause this dummy processing part can be polished at the same rate asthe synthetic-quartz-glass plate being processed and the fine particlesgenerated by the polishing can be prevented from marring the photomasksubstrate during the processing. However, the dummy processing part maybe made of a resin having the same properties.

The polishing apparatus to be used for polishing the raw platepreferably has such a size that at least one such raw plate can beplaced within the radius of the carrier or has such a size that thedummy processing part can be wholly held within the radius of thecarrier. This is intended to minimize the influence of a difference inpolishing rate between the central part and peripheral part of thecarrier.

An example of those processes is explained below. A quartz glass ingotsynthesized by a known method is cut into a given thickness with aninner diameter saw slicer. Thereafter, the glass plate obtained isbeveled with a commercial NC beveling machine so as to result in givenoutside dimensions and radiused edges.

Subsequently, this synthetic-quartz-glass plate is immersed in a 5% byweight HF solution in order to prevent the cracks generated by thecutting and the cracks generated by the beveling from propagating. Thissynthetic-quartz-glass plate is then lapped to a given thickness with aboth-side-lapping machine using an abrasive slurry.

The synthetic-quartz-glass plate thus lapped is subjected to the sameetching treatment as described above. Subsequently, thissynthetic-quartz-glass plate is polished with a slurry containing ceriumoxide as a main component and a polyurethane pad using aboth-side-polishing machine and then subjected to finish polishing witha slurry containing silica sol as a main component and a foamedpolyurethane pad using the same type of machine. Thus, a photomasksubstrate having a given thickness is obtained.

The pattern formation side preferably has a surface roughness of 0.3 nmor less in terms of Rrms value. The term “rms” stands for“Root-Mean-Square” and it represents a square root of the mean ofsquared values of deviations with respect to the average value.Referring to one-dimensional case, for example, the surface roughnessRrms value is given by the following formula:Rrms=√(Σ_(i)(f(x _(i))−m)² /n)wherein f(x) represents a cross-sectional profile of the surface shapeimage in x-axis direction, m represents an arithmetic mean of f(x), andn represents the number of points of surface roughness measurement.Thus, the rectilinear propagation of the light which has passed throughthe pattern on the photomask substrate can be easily secured even whenpolarized illumination is used or immersion exposure is conducted. Inaddition, defects of the kinds described above which have a size of 150nm or larger can be easily detected by irradiation with scattered light.

The photomask substrate produced in the manner described above can havea birefringence, as measured at a wavelength of 193 nm, of 0.5 nm orless per thickness of the substrate. Although birefringence generally ismeasured with an He—Ne laser having a wavelength of 633 nm, this foundvalue can be converted to the birefringence at wavelength of 193 nm bymultiplying that value by 1.32. Incidentally, the thickness of thephotomask substrate typically is about 6.35 mm.

The mask blanks of the invention are produced by superposing alight-shielding film on a surface of the photomask substrate obtained inthe manner described above. As the light-shielding film is generallyused a thin metal film made of chromium. The thickness thereof istypically 100 to 160 nm.

This thin metal film made of chromium can be formed by sputtering in thefollowing manner. The photomask substrate and a target comprisingchromium as the main component are set in a film deposition chamber. Thechamber is evacuated to a high vacuum to sufficiently discharge residualgases from the apparatus. Thereafter, while evacuating the chamber witha vacuum pump, a rare gas such as argon is introduced to form areduced-pressure atmosphere whose pressure is kept at a given value byregulating the gas flow rate or evacuation rate. In thisreduced-pressure atmosphere, a negative high voltage is applied to thecathode to generate a glow discharge. The glow discharge yields rare-gasions, which are accelerated by the cathode voltage and impinge andcollide against the target. Chromium atoms are thus dislodged from thetarget and are deposited on the substrate to thereby form a thin film.Examples of the glow discharge include a direct-current dischargegenerated by application of a direct-current voltage and ahigh-frequency discharge generated by application of a high-frequencyvoltage. Although either of these can be used, it is preferred to use adirect-current discharge formed by application of a direct-currentvoltage because the application of a high voltage can be easilyconducted stably and the glow discharge can be concentrated around thetarget. It is also preferred to use a direct-current pulse discharge forthe purpose of inhibiting the generation of an abnormal discharge orimproving suitability for the control of film deposition conditions.

There are various techniques of sputtering. However, the magnetronsputtering method, in which a magnetic field is used to heighten theplasma density around the target, is preferred because a film havingexcellent uniformity in film thickness and homogeneity can be formedwith satisfactory productivity. The explanation given below is on themagnetron sputtering method unless otherwise indicated. Besides themagnetron sputtering method, the ion beam sputtering method may, forexample, be used in which an ion beam generated by an ion gun is causedto impinge on the target and target atoms thus dislodged from the targetare deposited on a substrate. In this case, the sputtering atmospheregas is less apt to come into the film being deposited. This methodfurther has an advantage that film thickness and film homogeneity can becontrolled highly satisfactorily.

Argon, which is inexpensive, is frequently employed as the rare gas foruse as the sputtering atmosphere. Although argon is preferred, use maybe made of helium, neon, krypton, xenon, or the like.

It is known that a chromium film formed by sputtering is generally in atensile-stress state because of a film stress due to the structuraldefects and holes generated during film deposition. In case where thelight-shielding film has a high film stress, this warps the substrate,resulting in a birefringence. It is therefore preferred to reduce thefilm stress in the light-shielding film.

Film stress is known to vary depending on the conditions under which thefilm is deposited by sputtering. Namely, by suitably selecting filmdeposition conditions, a light-shielding film can be formed whileregulating the film stress so as to be in a desired range. Methods forreducing the film stress of a light-shielding film are explained below.

It is known that properties of the film to be deposited by sputteringwith a glow discharge vary depending on the pressure of the atmospherein which the sputtering is conducted (hereinafter referred to assputtering pressure). In the deposition of a chromium film bysputtering, a sputtering pressure on the order of 10⁻¹ Pa results in atensile stress of 1 to 2 GPa. The stress decreases with decreasingsputtering pressure, and an even lower pressure results in a compressivestress. Consequently, the sputtering pressure is preferably regulated to1.0×10⁻² to 1.0×10⁻¹ Pa. On the other hand, excessively reducedsputtering pressures may pose a process problem such as an unstable glowdischarge or a reduced film deposition rate. Consequently, thesputtering pressure is preferably regulated to 2.0×10⁻² to 8.0×10⁻² Pafrom the standpoints of attaining a more reduced tensile stress andavoiding such process problems. Furthermore, the residual gas in theapparatus is causative of property fluctuations of the film deposited.It is therefore preferred for property stabilization that the filmdeposition chamber be evacuated to a vacuum of at least 1×10⁻³ Pa priorto film deposition. More preferably, the chamber is evaluated to avacuum higher than 1×10⁻⁴ Pa.

The structural defects and holes which are generated during filmdeposition can be controlled by regulating the composition of the film,i.e., by adding one or more other ingredients. By adding oxygen ornitrogen to the argon as the sputtering atmosphere gas, the tensilestress can be greatly reduced. On the other hand, addition of oxygen ornitrogen in too large an amount may result in a film which has reducedlight-shielding properties and is hence unsuitable for use as alight-shielding film. Consequently, the proportion of the flow rate ofoxygen to the overall gas flow rate, i.e., the total flow rate of argonand oxygen, is preferably regulated to 30% or lower, and is 25% or lowerfrom the standpoint of obtaining sufficient light-shielding properties.In the case of adding nitrogen, the proportion of the flow rate ofnitrogen to the overall gas flow rate, i.e., the total flow rate ofargon and nitrogen, is regulated to preferably 30% or lower, morepreferably 20% or lower. In the case of simultaneously adding oxygen andnitrogen, the total addition amount thereof is preferably 30% or less,more preferably 20% or less, in terms of the proportion of the totalflow rate of oxygen and nitrogen to the overall gas flow rate, i.e., thetotal flow rate of argon, oxygen, and nitrogen. The lower limit of theamount of each of oxygen and nitrogen to be added is preferably 5% fromthe standpoint of sufficiently obtaining the effect of the addition.

A reduction in tensile stress can be attained also by using an alloytarget comprising chromium and one or more other elements in place ofthe chromium target to deposit a chromium alloy film as alight-shielding film. The alloying ingredients to be used preferably aremetals which, through sputtering, give a film having a compressivestress. Namely, it is preferred to use a chromium alloy targetcontaining one or more metals selected from the group consisting of Mo,Ta, Nb, W, Ti, and Zr as alloying ingredients. The total amount of suchalloying ingredients to be added is preferably regulated so as to be inthe range of 10 to 40 atomic % in terms of the proportion of theingredients to the number of all atoms constituting the target excludingoxygen and nitrogen. In case where the amount of the alloyingingredients is smaller than 10%, a sufficient stress-reducing effect isnot obtained. On the other hand, amounts thereof exceeding 40% result ina possibility that chemical resistance required of photomask blanksmight be reduced. It is also preferred that carbon or boron be added asan alloying ingredient to the chromium in place of or simultaneouslywith the alloying ingredients described above. Carbon and boron can beadded as alloying ingredients to the target like the alloyingingredients described above. However, in the case of carbon, inparticular, a gas containing carbon can be added to a sputteringatmosphere gas to deposit a film containing carbon incorporated therein.Preferred examples of the gas containing carbon include CO₂ and CH₄. Theamount of the carbon-containing gas to be added is preferably 30% orsmaller, more preferably 20% or smaller, in terms of the proportion ofthe flow rate of the carbon-containing gas to the overall gas flow rate,i.e., the total flow rate of argon and the carbon-containing gas.

Also preferred is a technique in which in place of or in combinationwith the formation of a chromium alloy film, the chromium-based filmhaving a tensile stress and a metal film having a compressive stress arelaminated on each other to cause the two stresses to countervail eachother. Examples of the metal constituting the metal film having acompressive stress include Mo, Ta, Nb, W, Ti, and Zr. One chromium filmand one metal film made of any of such metals may be laminated, or thesetwo kinds of films may be alternately laminated to form a film composedof many layers.

It is preferred that the chromium film or chromium alloy film formed orthe multilayer film comprising a chromium film and a film of anothermetal be subjected to a heat treatment to thereby mitigate structuraldefects and holes in the film and reduce the stress. This heat treatmentmay be accomplished by holding the film in an atmosphere such as dryair, nitrogen, or argon at 200 to 350° C. for 5 to 60 minutes.

The light-shielding film thus formed can have a film stress reduced to800 MPa or lower. As a result, the photomask substrate having thislight-shielding film laminated on a surface thereof can have a warpageamount reduced to 2 μm or smaller.

In the mask blank having the constitution described above, when alight-transmitting area of 260 nm×1,040 nm, which corresponds to thepattern of a gate electrode, is formed in the light-shielding film, thenthe birefringence at this light-transmitting area can be as low as 1 nmor less per substrate thickness as measured at a wavelength of 193 nm.In addition, the whole mask blank including the light-transmitting areacan have a birefringence as low as 1 nm or less per substrate thicknessas measured at a wavelength of 193 nm. Consequently, the photomaskproduced from this mask blank is suitable for use in the immersionexposure technique and the polarized illumination technique.

EXAMPLES

The present invention will be illustrated in greater detail withreference to the following Examples, but the invention should not beconstrued as being limited thereto.

Example 1

By the known soot method, SiCl₄ is hydrolyzed in an oxyhydrogen flameand the resultant fine SiO₂ particles are deposited on a target toproduce a porous quartz glass body of a cylindrical shape having adiameter of 35 cm and a height of 100 cm. This porous quartz glass bodyis placed in an electric furnace capable of atmosphere control. Whilethe atmosphere in the furnace is kept at a reduced pressure of 10 Pa orlower, the glass body is heated to 1,450° C. and held at thistemperature for 10 hours to produce a transparent synthetic quartz glassbody.

This transparent synthetic quartz glass body is placed in ahigh-temperature heating furnace equipped with a graphite heater. Theglass body is heated to 1,750° C. to deform it by its own weight andthereby mold it into a block product having dimensions of 17×17×25 cm.After completion of the molding, the block product is annealed bygradual cooling under vacuum. In this annealing, the block product isheld at 1,300° C. for 16 hours and then gradually cooled from 1,300° C.to 1,050° C. at a cooling rate of 2° C./hr.

A platy product having a length of 153 mm, width of 153 mm, andthickness of 6.4 mm is cut out of the block product obtained, and isplaced in an electric furnace capable of atmosphere control. This platyproduct is subjected to a hydrogenation treatment by holding it at 500°C. in a 10 to 100% hydrogen atmosphere of 1 to 10 atm. Thereafter, theplaty product is polished to produce a photomask substrate.

This photomask substrate is examined for the concentration of OH groupsand the concentration of oxygen-deficient defects by the methodsdescribed above. The concentration of OH groups is 78 ppm and theconcentration of oxygen-deficient defects is 5×10¹⁴ defects per cm³ orlower. Furthermore, the photomask substrate is examined forbirefringence, fluorine concentration, H₂ concentration, and Xe excimerlamp resistance by the methods which will be described later.

Subsequently, a light-shielding chromium film is formed by the magnetronsputtering method on a surface of the photomask substrate obtained. Achromium target having a diameter of 30 cm and a thickness of 5 mm isattached to the magnetron cathode in the vacuum chamber of a filmdeposition apparatus. The photomask substrate 6 inches square and 6.35mm thick is set on the substrate stage in the chamber. The distancebetween the target and the substrate is adjusted to 20 cm. The vacuumchamber is evacuated roughly and then to a high vacuum of 10⁻⁴ Pa or alower pressure. Thereafter, 30 sccm argon gas is introduced with a gasintroduction system while evacuating the chamber with a turbo-pump, andthe evacuation conductance is regulated to adjust the pressure in thechamber to 7.0×10⁻² Pa. Subsequently, a direct-current voltage of 2.5-kWconstant power is applied to the cathode from an external sputteringpower source to generate a glow discharge. Thus, a light-shieldingchromium film having a thickness of 100 nm is deposited. The film stressof the light-shielding film and the warpage amount of the substrate aremeasured by the methods which will be described later.

Example 2

On a surface of a photomask substrate produced in the same manner as inExample 1 is formed a light-shielding film of nitrogen-doped chromiumwith the same apparatus as in Example 1 in the same manner. In thisExample, 24.0 sccm argon gas and 6.0 sccm nitrogen gas are introduced inplace of the argon gas alone in Example 1. After the chamber isevacuated to 2×10⁻⁴ Pa, these gases are introduced into the chamber withthe gas introduction system and the evacuation conductance is regulatedto thereby adjust the pressure in the chamber to 7.0×10⁻² Pa. Sputteringis conducted while applying a direct-current voltage of 2.5-kW constantpower to deposit a nitrogen-doped chromium film having a thickness of150 nm on the substrate. The film stress of the light-shielding film andthe warpage amount of the substrate are measured by the methods whichwill be described later.

Evaluation

Evaluation Method 1: Birefringence of Photomask Substrate

Each of 121 points selected in the photomask substrate which aredistributed in a lattice pattern at an interval of 14.2 mm in a centralarea of 142×142 mm in the substrate was examined for birefringence withEXICOR, manufactured by HINDS Instruments, Inc., which employs an He—Nelaser as a light source. The maximum value of birefringence isdetermined. The birefringence of the photomask substrate of Example 1 is0.38 nm per 6.35 mm, which is the thickness of the substrate.

Evaluation Method 2: Fluorine Concentration

A sample having dimensions of 15 mm×15 mm×6.3 mm is cut out of a centralpart of the photomask substrate and examined for fluorine concentrationby the following method.

According to the method described in Nippon Kagakukai-shi, 1972(2), 350,the sample is thermally melted with anhydrous sodium carbonate, anddistilled water and hydrochloric acid (1:1) are added to the resultantmelt to prepare a sample liquid. The electromotive force of this sampleliquid is measured with a radiometer using a fluorine-ion-selectiveelectrode and using each of No. 945-220 and No. 945-468, bothmanufactured by Radiometer Trading K.K., as a reference electrode. Theconcentration of fluorine is determined based on a calibration curveobtained beforehand using standard fluorine ion solutions. The detectionlimit for this method is 10 ppm. The fluorine concentration in thesubstrate of Example 1 is 389 ppm.

Evaluation Method 3: Hydrogen Molecule Concentration

The photomask substrate is analyzed by Raman spectrometry. Theconcentration of hydrogen molecules (molecules/cm³) is determined fromthe intensity ratio (=I₄₁₃₅/I₈₀₀) between the intensity of thescattering peak at 4,135 cm⁻¹ (I₄₁₃₅) and the intensity of thescattering peak at 800 cm⁻¹ attributable to the fundamental vibration ofsilicon and oxygen (I₈₀₀) in the laser Raman spectrum (V. S.Khotimchenko et al., Zhurnal Prikladnoi Spektroskopii, Vol. 46, pp.987-997, 1986). The detection limit for this method is 1×10¹⁶ moleculesper cm³. The hydrogen molecule concentration in the substrate of Example1 is lower than 2.9×10¹⁸ molecules per cm³.

Evaluation Method 4: Suitability for Lithography Light Source

The photomask substrate is irradiated using an Xe excimer lamp of 13.2mW/cm² for 20 minutes. The decrease in light transmittance as measuredat a wavelength of 217 nm which has resulted from the Xe excimer lampirradiation is determined to evaluate the suitability. The decrease inlight transmittance of the substrate of Example 1 is 0.092%. Thechlorine concentration therein is 10 ppm or lower.

Evaluation Method 5: Film Stress

The film stress of the thin chromium film can be determined by X-raydiffractometry. Namely, the lattice constant d of the crystals in thethin chromium film on the substrate is determined by X-raydiffractometry. From the difference Δd between the thus-determinedlattice constant d of the thin chromium film and the lattice constant d₀of the bulk material, the lattice distortion in the thin-film thicknessdirection (ε=Δd/d₀) is determined. The film stress σ which is thein-plane stress of the thin film can be determined from that latticedistortion ε and the Young's modulus E and Poisson's ratio ν of the thinfilm, i.e., determined using the relationship σ=Eε/2ν. The chromiumfilms in Examples 1 and 2 each have a tensile stress of 800 MPa orlower. Besides being determined by the method based on X-raydiffractometry, the stress of the light-shielding chromium film may bedetermined by a method in which the warpage of the substrate is examinedwith an optical interferometer before and after the film formation andthe stress is determined from the change thereof.

Evaluation Method 6: Warpage Amount

The photomask substrate on which a light-shielding film has beenlaminated is evaluated for warpage amount. The substrates of Examples 1and 2 each have a warpage amount of 2 μm or less.

Evaluation Method 7: Birefringence of Mask Blank

The birefringence of the photomask substrate on which a light-shieldingfilm has been laminated is determined by examining a phase differencebetween a reference light and a reflected light using an He—Ne laser.The photomask substrates of Examples 1 and 2 each have a birefringenceof 0.8 nm or less per 6.35 mm, which is the thickness of the substrate.

INDUSTRIAL APPLICABILITY

The mask blanks of the invention have a low birefringence and aresuitable for use in the immersion exposure technique and the polarizedillumination technique.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

This application is based on Japanese Patent Application No. 2005-33098filed Feb. 9, 2005, the contents thereof being herein incorporated byreference.

1. A mask blank which comprises a substrate made of a synthetic quartzglass and a light-shielding film laminated on a surface of the substrateand is for use in a semiconductor device production technique employingan exposure light wavelength of 200 nm or shorter, wherein the maskblank has a birefringence, as measured at a wavelength of 193 nm, of 1nm or less per substrate thickness.
 2. A mask blank which comprises asubstrate made of a synthetic quartz glass and a light-shielding filmlaminated on a surface of the substrate and is for use in asemiconductor device production technique employing an exposure lightwavelength of 200 nm or shorter, wherein when a light-transmitting areaof 260 nm×1,040 nm is formed in the light-shielding film, then thebirefringence at this light-transmitting area, as measured at awavelength of 193 nm, is 1 nm or less per substrate thickness.
 3. A maskblank which comprises a substrate made of a synthetic quartz glass and alight-shielding film laminated on a surface of the substrate and is foruse in a semiconductor device production technique employing an exposurelight wavelength of 200 nm or shorter, wherein the substrate has abirefringence, as measured at a wavelength of 193 nm, of 0.5 nm or lessper substrate thickness, and wherein the light-shielding film has a filmstress of 800 MPa or lower.
 4. A mask blank which comprises a substratemade of a synthetic quartz glass and a light-shielding film laminated ona surface of the substrate and is for use in a semiconductor deviceproduction technique employing an exposure light wavelength of 200 nm orshorter, wherein the substrate has a birefringence, as measured at awavelength of 193 nm, of 0.5 nm or less per substrate thickness, andwherein the mask blank has a warpage amount of 2 μm or smaller.
 5. Themask blank of claim 1, wherein the synthetic quartz glass has an OHgroup concentration of 100 ppm or lower.
 6. The mask blank of claim 1,wherein the synthetic quartz glass has a fluorine concentration of 100to 10,000 ppm.
 7. The mask blank of claim 1, wherein the syntheticquartz glass has a concentration of oxygen-deficient defects of 5×10¹⁴defects per cm³ or lower.
 8. The mask blank of claim 1, wherein thesynthetic quartz glass has an OH group concentration of 100 ppm or lowerand a fluorine concentration of 100 to 10,000 ppm.